Thursday, June 18, 2015

Hot Dog!

As mentioned a while ago, I've been awarded a slot on the 757/767, but training isn't until late October and that means I get another four months on the Mad Dog. I just had lunch with a couple of guys down in Atlanta yesterday, and though they now all fly the 757/767 one noted that he had spent "five summers on the Mad Dog...because you always measure time on that airplane in summers!" I suggested that if he's ever really bored in cruise, he should attempt to calculate how many gallons of sweat he shed over those five summers. Or not - I guess that's kinda gross.

The basic problem is that as the Mad Dog was stretched from earlier airplanes, the Auxiliary Power Unit and Air Cycle Machines ("packs") were not also upgraded to accommodate the increased cabin volume. In moderate temperatures or with both engines running at high power to put out a lot of bleed air, it's not really a problem. On the ground with a high OAT, though, both the amount of airflow and output temperature are wholly inadequate. The cockpit is a bit worse than the cabin, because you have a lot of windows, the old CRTs, electronics, and incandescent lighting put out a lot of heat, and you're sitting 120 feet forward of the packs with resulting efficiency losses through the ductwork. At times it can get miserably hot in the cockpit, especially when you're busy just before pushback. In the summer you just about have to pack a new uniform shirt & undershirt for every day of the trip.

Out of my duties as a Mad Dog FO is running the AC, largely because most of the controls are on the right side of the overhead panel. There are some tips and tricks to getting the most out of the system, and most FOs get pretty good at it. The primary controls are the packs supply switches with OFF, AUTO, and HP BLEED OFF positions, and two selector knobs, one for CKPT TEMP and one for CABIN TEMP. The indicators are L and R valve position gauges and flow gauges, as well as a larger temperature gauge that indicates either cabin temp or cabin supply duct temperature, depending on the position of a TEMP SEL knob. Most of the time this is left in the CABIN SPLY position, which along with the valve indicators is a good measure of what the system is doing at any given moment; every couple of minutes I'll flip the TEMP SEL knob over to Cabin to see how we're doing overall.

We nearly always leave the Supply switches in the AUTO position except when parked at the gate with APU off and the conditioned air hose attached to the aircraft, at which point we turn them OFF. The HP BLD OFF ("High Pressure Bleed Off") position is rarely used, though there are circumstances when it can slightly increase airflow. The temp selector switches can also usually be left in the AUTO range, which commands the system to try to attain and maintain a specific cockpit and cabin temperature. The problem is that the exact temperature you're commanding is not labeled, and every airplane is different! Most airplanes have little ink marks that previous FOs have added to show a position that works, and this makes a good starting point, but there are often multiple marks as the system changes over time with wear, maintenance and repairs. Sometimes it seems to change over the course of a single flight!

On the ground with OATs warmer than about 60º F, we leave both selector knobs pointing at about the 9 o'clock position, which commands both valves full cold and gives you about as cold of air as you can expect (sometimes adjusting them slightly higher can increase airflow, however). After takeoff, we turn the cockpit selector knob to around the 10:30 position and the cabin selector knob to slightly below whichever ink mark we've decided to use as a reference. The idea is to get the cockpit valve slightly above full cold, and the cabin valve right around the first index mark (as shown in the picture above). You want the cabin supply temperature to come off the bottom peg (lest you freeze the passengers out before decreasing the overall cabin temp to comfortable levels); I've found an initial supply temp of 50-60ºF works well. As the cabin temp comes down to the desired level (I shoot for 68° or so), I adjust the temp knob slightly higher into auto range - usually right around the reference mark - where ideally the system will modulate the supply valve to maintain a comfortable temp on its own (80-90° supply temp works well). Any further adjustments to the temperature knob are best made slowly and in small increments, lest you drive the valve full hot or cold. I've found that in about 75% of the fleet, you'll end up turning the cabin temp selector slightly lower over the course of the flight, and the cockpit temp selector slightly higher.

Sometimes AUTO mode just isn't working no matter what you do, and then we turn the affected temp selector knob to the six-o'clock position, which puts the respective pack in manual mode. At this point you are directly driving the supply valve with momentary selections hotter or colder; however, output temperature can still vary depending on engine power / bleed output, and more than one pilot has left a pack in MANUAL mode, didn't pay attention when the throttles came back on descent, and was surprised to find that the output temperature was driven so high that it tripped the pack offline altogether. The preferred technique if MANUAL mode is needed is to get the supply valve where you want it, let output temp stabilize for a minute or two, and then switch back to AUTO mode and see if it does a better job of maintaining supply valve position.

On descent, depending on destination OAT, you'll typically once again turn both knobs to the 9 o'clock position in an attempt to cold-soak the cabin before arrival - as temps are nearly guaranteed to rise on taxi-in. If it's very warm at all, I start the APU immediately after landing and open the crossbleed valves so it can assist with airflow. We normally shut down the #2 engine three minutes after landing for fuel savings, but with OAT over 85ºF we occasionally keep both engines running to the gate (especially in an airplane that has trouble keeping up on a single idling engine + APU). Once both engines are shut down at the gate, the cabin temperature is almost guaranteed to climb on APU alone; it's basically a question of how quickly you can get the warm bodies to exit the airplane! Nearly all of our gates have large and capable air conditioning machines with supply hoses, and the better rampers get these hooked up and running very quickly. In warm weather they almost always do. The problem is that in 60º-70º weather they often don't realize that this conditioned air is still very necessary on the Mad Dog. The proactive pilot will go outside and make sure they still hook up the air. Once this is complete, you can save fuel by shutting down the APU in all but the hottest temperatures.

This all applies to the normal Mad Dog. On the Big Dog, which makes up about 30% of our fleet, the controls are the same but the actual system components are different and require a different technique. The good news is that the APU is bigger and has more bleed output, and is often able to maintain a comfortable cabin temperature on the ground all on its own or with a single engine running. The bad news is that the system is far more aggressive and volatile in AUTO mode. It will run the supply valves from full cold to full hot with a tiny movement of the temperature selector knob, or sometimes all on its own. The initial after takoff setting of the knobs requires particularly close attention for the first several minutes, meaning that at a fairly busy time I'm looking up at the overhead panel every 30 seconds or so. It's far more common to have to run for a while in manual mode on the Big Dog.

This all sounds fairly labor intensive, and it is when you're new to the system, but once you're acquainted with its quirks it becomes second nature, much like the rest of the Mad Dog. I have a feeling that once I go to the 757/767 I'm going to initially be a little restless, feeling like I ought to be doing far more than I am. I'm sure I'll get used to the reduced workload, though - not to mention skating through summer as cool as a cucumber!

8
comments:

Anonymous
said...

Thanks for a really informative post. Living in Florida, I'm used to boarding really hot aircraft and wondering why they can't be cooled. While it will still be uncomfortably warm, at least now I understand much better why that is. As is often the case, lots of things are being done to keep us happy and we're completely unaware of how much work that can take.

As an airplane mechanic who works on mad dogs some, its very endearing to read about the ink marks and all other quirks:) Nice work, very few pax know how much effort it takes to fine tune it from the flight deck

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Hey Sam! I read your blog years ago when you were with Horizon. I'm glad to see that you're still writing and doing well. Quite well from the sounds of it! If you're ever in MAD on an overnight, let me know and I'll treat to some tapas.